Owing to the urgent need for fuel-saving engines, direct-injection Otto-cycle engines using lean combustion methods are currently being developed. The new generation of engines can achieve a mean fuel saving of up to 15%. Charge stratification is carried out in the combustion chamber when on partial load. This means that the combustion chamber is split into two zones, a first zone with a fuel/air mixture which is capable of being ignited in the vicinity of the spark plugs, and a second zone composed of air and residual gas, which surrounds the first zone and is thermally isolated from the walls of the combustion chamber. Stratification charging is dependent on an extremely late injection time during the compression phase of the engine, and on an extremely short injection time of typically 0.5 ms. As the engine load increases, a transition takes place to homogeneous operation. In this case, the fuel is actually injected during the induction phase, that is to say very early, in order to ensure good, thorough mixing of the air and fuel.
It is particularly advantageous to use piezo actuators or other electrostrictive actuators manufactured using a multilayer technique for operation of the injection valves, since they react with virtually no delay. The piezo actuators or electrostrictive actuators manufactured using a multilayer technique have a layer stack composed of a material whose extent changes when an external voltage is applied in the longitudinal direction. Injection valves which are operated by piezo actuators or electrostrictive actuators can be controlled independently of the piston movement and, furthermore, have the advantage that they can be used to achieve short switching times.
For circuitry purposes, the piezo actuator represents a capacitance which is charged by means of an external applied electrical voltage. Energy is therefore stored in the piezo actuator. Piezo actuators use, for example, switching processes at frequencies of between 10 and 500 Hz for charging and discharging.
The German patent application with the official file reference 101 47 168.8 describes a converter circuit by means of which the energy which is stored in the secondary energy storage capacitance can at least partially be transferred back to the primary energy storage capacitance. This is achieved by additionally fitting a primary energy storage capacitance on one side. The energy which is stored in the secondary energy storage capacitance can result in a current being built up via the energy storage inductance and the secondary energy storage capacitance to this primary energy storage capacitance, in particular by briefly closing a secondary switching element, which current flows again when the secondary switching element is subsequently opened after a short time, and thus charges the primary energy storage capacitance.
Since, for many applications, the power output stage shall be designed to be as compact as possible, all the components should be checked for volume reduction. However, the power output stages circuit that is shown in FIG. 2, corresponding to the prior art, requires each component to have specific minimum characteristic values, for internal matching. As before, the largest amount of space is in this case consumed by the inductances and capacitances. If, for example, the primary energy storage capacitance is reduced by reducing the nominal capacitance, then this leads to greater potential fluctuation during the charging and discharging processes, which in turn would result in a filter inductor with a higher current load.